1. Field of Invention
The invention relates to a femoral hip-joint prosthesis having a tapered stem, for implantation by cement.
2. Description of Related Art
Laid-open patent application WO 91/18561 A1 discloses a femoral hip-joint prosthesis which is designed as a wedge-shaped intramedullary stem with a collarless shoulder. In order to secure a stem of this kind in the canal of a femur, a type of bone cement is used which normally comprises a mixture of polymethyl methacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer, and optionally contains a styrene copolymer of PMMA. These and other types of cement used for the purpose of securing the stem in the canal of the femoral bone are subject to a phenomenon known as creeping. Although the bone cement in the hardened state appears to be rigid, it is subject to tiny movements over the course of time, and these lead to disturbance of the microscopic denticulations constituting the cement/implant interface and the cement/bone interface. This effect can cause the stem to loosen with time. The known stem is designed in such a way that there is slight adhesion between the bone cement and the surface of the stem. If creeping of bone cement occurs, then the wedge-shaped stem easily sinks in the bone cement and automatically settles again within the bone cement. In order to prevent loosening of the stem, it is crucially important that as far as possible the creeping of the cement and sinking of the stem do not disturb the microscopic denticulations of the cement/bone interface.
A disadvantage of the known stem is the fact that because of the sinking of the stem in the femur, the pressure force exerted on the bone cement increases, particularly at the cement/implant interface, but also at the cement/bone interface, and in this case excess creeping or fissuring of the bone cement occurs in particular at those points where there is a great increase in pressure force.
The object of the present invention is to develop a femoral hip-joint prosthesis for implantation by cement in such a way that the bone cement is exposed to less stress, and in particular in such a way that there is minimal disturbance of the microscopic denticulations at the cement/bone interface.
This object is achieved with a femoral hip-joint prosthesis having a tapered stem for implantation by cement as described herein. Further advantageous embodiments of the invention are also described herein using the tapered stem femoral hip-prosthesis for implantation by cement having a logarithmically extending curvature in a first subsidiary section and a converging straight line in a second subsidiary section.
The invention is achieved in particular with a femoral hip-joint prosthesis for implantation by cement, having a collarless shoulder at the proximal end and having a stem which extends in a straight line from the shoulder to the distal end and tapers from the proximal end toward the distal end, the stem having a quadrilateral cross section and an anterior side face and a posterior side face, and the two side faces, in the direction of extension of the stem, having, in a first subsidiary section beginning at the shoulder, a logarithmically extending curvature, and the two side faces thereafter, in a second subsidiary section, extending in a straight line and converging.
The femoral hip-joint prosthesis according to the invention, hereinafter also referred to as a femoral stem, has an anterior side face and a posterior side face which, in the longitudinal direction, in a first subsidiary section beginning at the shoulder, each have a logarithmically extending curvature so that the cross section of the stem tapers toward the distal end. The anterior side face and the posterior side face each have, following the first subsidiary section, a second subsidiary section, which second subsidiary sections extend in a straight. line and converge toward the distal end.
The femoral stem according to the invention has the advantage that its configuration in the longitudinal direction corresponds approximately to the anatomical course of the medullary canal of the femur which has a trumpet-shaped structure in a first, upper subsidiary section, whereas the medullary canal, in a second, lower subsidiary section, has side walls which extend in straight lines, slightly narrowing in the distal direction. In a preferred embodiment, the femoral stem has a configuration which is adapted to the anatomical course of the medullary canal in such a way that the bone cement coming to lie between the stem and the femur has an approximately constant layer thickness. On account of the logarithmically extending curvature, the femoral stem according to the invention additionally has such a configuration that upon sinking of the femoral stem, the cement, particularly in the first, upper subsidiary section, experiences as uniform as possible a compression, which in the first subsidiary section effects an approximately uniform increase in force oriented perpendicular to the direction of extension of the femoral stem, so that stress peaks or fissures in the bone cement are avoided.
The advantage of this logarithmically extending configuration can of course also be exploited, in the first subsidiary section, by means of correspondingly logarithmically curved lateral and medial sides of the femoral hip-joint prosthesis.
A further advantage of the design of the femoral hip-joint prosthesis according to the invention lies in the fact that by avoiding stress peaks in the bone cement, the cement/bone interface has a fairly uniform distributed pressure load. It is known that in the case of a cement/bone interface with a non-uniform pressure load, regression of the bone can occur at areas where the pressure loads are high, which could result in additional loosening of the femoral stem.
The femoral stem according to the invention has a quadrilateral cross section with edges, the cross section being in particular square, rectangular or trapezoidal. It can prove advantageous to round off these edges in order to reduce the notch effect which these edges exert on the bone cement upon sinking of the femoral stem. In addition to the notch effect, the stress peaks acting on the bone cement are of course also reduced. In an advantageous design, the edges have a radius of between 1 and 3 mm. The hip-joint prosthesis according to the invention has the further advantage that because of the quadrilateral cross section of the stem, torsional forces or shearing forces acting on the femoral stem can advantageously be transmitted to the bone cement. The quadrilateral design of the cross section of the stem and the rounding-off of the edges permit optimum functioning with respect to the two opposing requirements below:
A femoral stem with many edges leads to high stress peaks as it sinks, or even to stress fractures in the cement, so that a round cross section would be best.
On the other hand, a round cross section would generate a high shear stress in the bone cement in the event of torsional forces acting on the femoral stem, so that a cross section with corners would be more advantageous.
In a preferred embodiment, the femoral stem has a highly polished surface, as a result of which the adhesion of the cement at the cement/implant interface is reduced or avoided. The action of the shearing forces which occur along the stem as the femoral stem sinks, or also the shearing forces which occur in the cement upon torsion or bending of the stem, are additionally greatly reduced in this way.
An illustrative embodiment of the femoral hip-joint prosthesis according to the invention is explained below with reference to FIGS. 1 through 3.